The effects of dynorphin-A (Dyn) and [Leu5]-enkephalin (Enk) were compared in the vitro hippocampal slice preparation, using extracellular field potential and intracellular voltage recordings. In the CA1 region, Dyn, like Enk, consistently increased the size of the extracellularly recorded population spike (PS) evoked by stratum radiatum (StR) stimulation of the Schaffer collaterals. These responses were naloxone reversible. In contrast, in the CA3 region, Dyn both increased and decreased the PSs evoked by mossy fiber stimulation, whereas Enk slightly enhanced the PS. Intracellular recordings from CA3 pyramidal neurons (HPNs) revealed both excitatory and inhibitory actions of Dyn on spontaneous activity. Associated membrane potential changes were variable. In contrast, Enk had only weak effects on spontaneous activity and no effect on membrane potential. These data suggest regional differences in the effect of Dyn and Enk on hippocampal activity.